1. Field of Invention
This invention relates to frequency shift keying systems and, more particularly, to such systems which can easily transmit at a selectable one of multiple carrier center frequencies.
2. Description of the Prior Art
With the ever increasing use of high density multichannel radio communications, the electromagnetic energy spectrum has become overcrowded. Thus, in order to avoid inter-channel interference, strict control of communication ranges in such communication systems is desirable. Moreover, since most medium and high output power transmitters perform with higher efficiency when operating at or near saturation of their input versus output amplitude transfer function, the ability to utilize transmitters with non-linear amplitude transfer functions has become an important design criteria in such communication systems.
Frequency shift keying (FSK), a technique of communicating digital information using discrete frequencies to represent specified symbols, has properties which are desirable in controlling the bandwidth of the system and for use with non-linear transmitters. Binary FSK, for example, transmits a first (mark) frequency to represent a binary one and a second (space) frequency to represent a binary zero. The amplitude of the carrier, in the ideal case, is invariant and thus can pass through non-linear amplifiers with minimal degradation of signal quality. Further, proper choice of the difference between the mark and space frequencies and the time of switching between these frequencies can provide strict control and minimization of the radiated spectrum thereby minimizing the required channel bandwidth and the spurious emission of power outside the channel bandwidth.
Prior art frequency shift keying systems are designed to transmit at a single carrier center frequency. There are, however, applications where transmission at multiple carrier frequencies in sequence with rapid changeability in frequencies is desired. In the prior art, multiple transmitter systems would be employed with each transmitter at a different carrier center frequency.